A hydraulic control system of an automatic power transmission for an automotive vehicle includes, in addition to a pressure regulator for producing a main control or line pressure, a transmission throttle valve which is adapted to produce a throttle pressure continuously variable with load on the engine with which the transmission is used together in an automotive vehicle. Among the transmission throttle valves presently in practical use, there is a throttle valve which is responsive to the movement of the accelerator pedal of a vehicle for producing a throttle pressure variable with the degree to which the throttle valve provided in the carburetor of the engine is open. A transmission throttle valve of this nature usually comprises a valve plunger connected to or engaged by the accelerator pedal through a mechanical linkage provided therebetween and a valve spool engaging the valve plunger through a spring provided between the valve spool and plunger. In the presence of a line pressure in one fluid inlet port of the throttle valve, the valve spool is not only subjected at one end thereof to the force of the spring but acted upon at the other end thereof by a fluid pressure which is developed by the valve spool attempting to attain a certain equilibrium position in which the force resulting from the fluid pressure urging the valve spool against the force of the spring is substantially equally balanced with the force of the spring. The mechanical linkage provided between the valve plunger and the accelerator pedal is arranged so that the valve plunger is continuously movable with the accelerator pedal. As the accelerator pedal is depressed from the released position thereof, the valve plunger is axially moved toward the valve spool and compress the spring provided between the valve spool and plunger. The force of the spring exerted on the valve spool is thus continuously variable with the distance of movement of the accelerator pedal and is therefore representative of the opening degree of the carburetor throttle valve. In other words, the throttle pressure developed by the transmission throttle valve is a function of the opening degree of the carburetor throttle valve and is variable as the accelerator pedal is moved between the released position and a fully depressed or kickdown position thereof. Usually, a transmission throttle valve having these functions is arranged so that the throttle pressure is reduced to zero level in response to the accelerator pedal in the released position and autgmented to a peak value close to the level of the line pressure when the accelerator pedal is depressed all the way down.
On the other hand, the pressure regulator valve provided in the hydraulic control system is arranged to be responsive to the throttle pressure thus developed by the transmission throttle valve so that the line pressure to be developed by the pressure regulator is also variable with the opening degree of the carburetor throttle valve. When the throttle pressure is maintained at zero with the accelerator pedal held in the released position, the pressure regulator produces a line pressure which is reduced to a certain minimum value. As the accelerator pedal is depressed and the throttle pressure developed by the transmission throttle valve increases, the line pressure delivered from the pressure regulator increases until the line pressure reaches the above mentioned peak value when the accelerator pedal is fully depressed.
In the event a failure is brought about in the mechanical linkage providing connection or engagement between the accelerator pedal and the valve plunger of the transmission throttle valve, the valve plunger becomes free to move independently of the accelerator pedal and as a consequence the spring provided between the valve plunger and the valve spool of the transmission throttle valve is disabled from imparting a force on the valve spool. When this occurs, the throttle pressure developed by the transmission throttle valve is reduced to zero level and as a consequence the line pressure developed by the pressure regulator drops to the above mentioned minimum level in spite of the fact that the engine is still in operation. If the vehicle must be driven to a repair shop under these conditions without being toward or otherwise aided by another vehicle, the fluid operated frictional units provided in the transmission mechanism are held operative by the line pressure which is reduced to the minumum value as above mentioned. The forces applied to the frictional units by such a line pressure are, however, far short of the loads required of the units so that the frictional units tend to produce undue slips between the friction elements thereof and will in the worst cases invite damages by heat.
With a view to solving these problems, it has been proposed and put into practice to increase the throttle pressure to the level of the line pressure in response to a failure brought about in the mechanical linage between the transmission throttle valve and the accelerator pedal. Such an expedient is on the one hand useful for avoiding damages of the frictional units of the transmission mechanism due to the reduction in the line pressure but results, on the other hand, in a problem that, since the throttle pressure augmented to the level of the line pressure is representative of a full throttle condition of the carburetor throttle valve, the hydraulic control system tends to make downshifts in response to part throttle conditions of the carburetor throttle valve. If, therefore, the vehicle is being driven with the transmission gear shift lever held in the automatic forward drive range position "D", there may be caused a sudden downshift to the first gear ratio in the particular range by reason of the upshifting tendency of the hydraulic control system of the power transmission. If this takes place, the vehicle will be accelerated suddenly or braked upon by the engine during cruising. Because, furthermore, an upshift can not be effected once a downshift has been made to the first gear ratio, the vehicle must be driven at limited speeds.
The present invention contemplates elimination of these drawbacks inherent in a transmission throttle valve of the type responsive to the movement of the accelerator pedal of an automotive vehicle.